This invention relates generally to communications systems, and, in particular to a receiver that performs adaptive channel equalization.
CDMA systems are based on a digital, wideband, spread spectrum technology which transmits, multiple, independent user signals across an allocated segment of the radio spectrum. In CDMA, each user signal includes a different orthogonal code and a pseudo random binary sequence that modulates a carrier, spreading the spectrum of the waveform, and thus allowing a large number of user signals to share the same frequency spectrum. The user signals are separated in the receiver with a correlator which allows only the signal with the selected orthogonal code to be despread. The other user signals, whose codes do not match, are not despread, and as such, contribute to system noise. The signal to noise ratio of the system is determined by the ratio of the desired signal power to the sum of all interfering signals, enhanced by the system processing gain and the ratio of the spread bandwidth to the baseband data rate.
This also applies to the 3rd generation wideband CDMA (WCDMA) standards currently proposed, where different spreading factors and variable user data rates can be supported simultaneously, that is, various users are multiplexed to the channel typically by orthogonal spreading codes. In CDMA systems in general, however, due to multipath propagation and frequency-selective fading, the orthogonality between the various users"" waveforms is degraded, and multiple access interference impairs the performance of the receiver.
The interference generated by the other user signals is called multiple access interference, and is essentially caused by the amount of user traffic in the channel, which corrupts the code orthogonality at the receiver.
Several multiuser detection schemes have been proposed to alleviate multiple access interference in the uplink of direct-sequence code division multiple access (DS-CDMA) systems. These methods, however, are too complex and burdensome to be incorporated into a conventional user terminal because they would negatively impact its power consumption and size.
Reference in this regard may be had to: A. Duel-Hallen, J. Holtzman, and Z. Zvonar, xe2x80x9cMultiuser detection for CDMA systems,xe2x80x9d IEEE Personal Communications, pp. 46-58, April 1995.
As an alternative, several adaptive algorithms with low computational complexity have been proposed to alleviate multiple access interference in CDMA systems. For example, in the presence of a known training sequence a least mean square (LMS) algorithm can be applied. However, when no training sequence is available, or training is impractical, blind adaptation schemes are better suited for CDMA interference reduction.
Reference in this regard may be had to: M. L. Honig, U. Madhow, and S. Verdxc3xa, xe2x80x9cBlind adaptive multiuser detection,xe2x80x9d IEEE Transactions on Information Theory, vol. 41, pp. 944-960, July 1995. Reference may also be had to: N. Zecevic and J. H. Reed, xe2x80x9cBlind adaptation algorithms for direct-sequence spread-spectrum CDMA single-user detection,xe2x80x9d IEEE International Vehicular Technology Conference, VTCxe2x80x297, May 1997, pp. 2133-2137.
However, most of the proposed adaptive interference suppression methods rely on the symbol level cyclostationarity of the signal. Unfortunately, this excludes their application to systems employing long scrambling codes, such as systems employing wideband CDMA (WCDMA), wherein the pseudorandom scrambling changes the correlation properties of the signal from symbol to symbol.
It is an object and advantage of this invention to provide a receiver which utilizes an adaptive channel equalization function to restore the orthogonality between synchronous users"" waveforms and to suppress other multiple access signals.
It is a further object and advantage of this invention to apply adaptive channel equalization to a CDMA system by first estimating a transmitted chip sequence, and which then utilizing an algorithm to converge to a linear minimum mean-square error (LMMSE) solution.
It is a further object and advantage of this invention to provide a CDMA receiver with improved performance which is suited for systems using long scrambling codes, such as third generation WCDMA systems.
It is a further object and advantage of this invention to provide a CDMA receiver with improved performance which does not require training sequences or training information, with the exception of ordinary channel estimation, which may be based on either dedicated pilot symbols, or a pilot channel.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
A receiver for use in a CDMA telecommunications system is disclosed. The receiver includes at least one antenna for receiving signals from a CDMA channel, an estimator for estimating an impulse response of the channel, and an adaptive equalizer. The estimator provides a reference for the adaptive equalizer, and the adaptive equalizer operates to estimate a transmitted chip sequence of the channel, to restore the orthogonality between synchronous users"" waveforms, and to suppress other multiple access signals. The adaptive equalizer includes circuitry for implementing a blind adaptive algorithm to estimate the transmitted chip sequence of the channel.
A receiver for use in a CDMA telecommunications system is disclosed. The receiver includes a least one antenna, an estimator for estimating an impulse response of a channel, and an adaptive equalizer. The estimator provides a reference for the adaptive equalizer, and the adaptive equalizer operates to equalize a signal in the channel by estimating the transmitted chip sequence. The effect of the adaptive linear receiver algorithm is to restore the orthogonality of the signal""s waveform.
The adaptive equalizer preferably utilizes a blind adaptive algorithm to estimate the transmitted chip sequence of the channel and to converge to a linear minimum mean-square error solution.